This invention addresses a problem that has remained unsolved since the first consideration of the concept in the 1960s.
John Coxon and Andrew Claughton disclosed the development of an auto-pitch propeller in the publication “The Development of a Self Pitching Propeller System”, (RINA), but the auto-pitch propeller uses the self-pitching technique without any damping torque. This means that each auto-pitch propeller blade rotates about its individual pivot axis freely (only with friction torque that is nearly zero). In operation, the suction side and pressure side of the auto-pitch propeller blades do not alternate.
Oscillating or flapping foil propulsors were inspired by the nature of marine swimmers. Two propulsion modes are mainly applied: the undulation mode and the thuniform mode. The undulation mode is seen in the propulsion of eels (low frequency and large amplitude for low speed swimmers). The thuniform mode is widely used by fast swimmers such as shark, dolphin, tuna and whale. An application example for the undulation mode is the Nektor™ thrust unit for unmanned underwater vehicles in which a flexible oscillating foil member rotates counterclockwise and clockwise, alternatively, about its pivoting axis to generate thrust. This mechanism is disclosed in U.S. Pat. No. 6,250,585 of Jun. 26, 2001.
In the Nektor™ unit the foil and the rotational shaft do not have relative rotation. The shaft rotates in counterclockwise and clockwise directions alternatively to create thrust. This propulsor uses the undulation mode (eel's propulsion mode) so it does not produce thrust if the foil is rigid.
A rigid oscillating foil member with a zero damping torque, about its pivot axis will produce nearly zero thrust. In the traditional oscillating foil configuration, the damping torque is infinity, i.e., the pivot shaft and the foil are the same piece. When a traditional rigid oscillating foil (infinite damping torque) has only heave motion, it produces nearly zero thrust at low advance speed. Enabling both pitch and heave motion for an oscillating foil requires a complicated control mechanism, which is too complicated and of low efficiency to be used in practical application. This invention provides a heave only oscillation foil with a regulated thrust at all speeds. At standstill, the start-up speed of a marine vehicle is important; a zero thrust at start up means that a vehicle will never start to move).
Regardless of the mode of oscillating foil propulsors, all previous oscillating foils have no relative angular displacement between the foil's pivot axis and the rotational shaft, i.e., there is no slip rotation. Pitch of the oscillating foil member is either controlled by a pitch mechanism, such as a flexible bar that is linked to the foil or there is no pitching at all (pure heave motion with a constant pitch angle). To increase the efficiency of propulsion, there have been some foil members and spans made with flexibility in the foil itself.